Secondary batteries and fuel cells are high-efficient, clean energy sources. In recent years, electric vehicles, fuel cell vehicles and trains equipped with such secondary batteries and fuel cells as power supplies have been under development on a worldwide basis.
Attention has been given to a fuel cell as a power supply having high energy conversion efficiency and a small environmental load. A fuel cell is incapable of accumulating electric power. However, it is possible to construct a certain power storage system by combining a fuel cell with, for example, a hydrogen producing apparatus for producing hydrogen by electrolysis of water. Such a power storage system is called a reversible fuel cell (refer to Patent Literature 1 and Patent Literature 2). The reversible fuel cell constructed by combining the fuel cell with the water electrolyzer performs water electrolysis, which is a reverse reaction of power generation, using natural energy or night-time electric power, during the reversible fuel cell is not generating electric power. Thus, this power generation system produces fuel for its own use.
On the other hand, a secondary battery has been used as a power supply for an electric or electronic device requiring large current discharge, such as an electric tool. In recent years, particularly, attention has been given to a nickel-metal hydride secondary battery and a lithium-ion secondary battery as a battery for a hybrid vehicle to be driven by an engine and the battery.
A typical secondary battery is charged with electric energy, thereby storing electricity. Patent Literature 3 discloses a secondary battery which is rechargeable with gas. Moreover, Patent Literature 4 discloses a new type fuel cell which is a combination of a fuel cell and a secondary battery and contains manganese hydroxide as a positive electrode active material and a hydrogen storage alloy as a negative electrode active material.